A non-aqueous electrolyte secondary battery such as a lithium ion secondary battery is widely used as an electric power source of portable electronic devices such as mobile phones, video cameras, and laptop computers. Further, it is increasingly used as an electric power source of electric cars, hybrid cars, and large electric storage devices.
At present, a liquid electrolyte wherein an electrolytic salt is solved in a nonaqueous solvent is used as an electrolyte of these non-aqueous electrolyte secondary batteries. However, the liquid electrolyte contains a flammable solvent and a liquid spill sometimes occurs. Therefore, a safety improvement is desired.
In order to improve the safety of lithium ion secondary battery, all solid secondary battery using a dry type solid electrolyte instead of a liquid electrolyte has been developed. In such all solid secondary battery, a flame-resistant ionic liquid, gelatinous electrolyte, and polymer solid electrolyte are examined as a dry type solid electrolyte. However, a practicable secondary battery using the dry type solid electrolyte exhibiting the same performance as liquid electrolyte cannot be obtained.
As mentioned above, the lithium ion secondary battery has a high voltage and a high capacity, and widely used as a secondary battery. However, an amount of deposit of lithium is not abundant, and therefore a magnesium ion secondary battery using a magnesium with abundant reserve is actively developed instead of the lithium ion secondary battery. The magnesium ion secondary battery is characterized in that it is easy to handle, and a theoretical electric capacity density per volume is high.
In fact, however, a practicable electrolytic solution capable of stably and safely charging and discharging has not been found, and therefore the magnesium ion secondary battery is not put into practical use. Further, it has not been reported that the magnesium ion secondary battery is activated using the dry type solid electrolyte.
A molten salt is a salt consisting of cation and anion. Further, it can conduct electricity without adding an electrolyte, and has a wide potential window and a high ion conductivity. Among the molten salts, molten salts having a melting point of 100 to 150° C. are referred to as ionic liquids, although it is not a strict classification. Further, molten salts having a comparatively high melting point, which has flexibility and is not in the crystal state but in the solid status, is sometimes referred to as plastic crystals. The molten salts having characters such as an ion conductivity, a slight volatility, a flame resistance, and a thermal stability, is expected to be applied to the electrolytes of secondary batteries or capacitors.
Molten salts have characters such as an ion conductivity, a slight-volatility, a flame resistance, and a thermal stability, which are suitable for use as the electrolyte, and thus it is considered that the safety of electrochemical devices such as secondary batteries is improved. However, the molten salt is liquid, and therefore, when the molten salt is used in the secondary battery, and the like, there is a problem of liquid spill therefrom.